AC motors for industrial applications typically exhibit a plurality of windings and operate on three phases of electricity. Generally speaking the windings are arranged symmetrically and connected to the three phases of electricity either in a delta configuration or a star configuration, with the star configuration also known as a Wye configuration. In the delta configuration, the finish of each winding is connected to the start of the next winding. In the star configuration, one end of all of the three windings are connected together. The delta configuration exhibits an increased current through the motor windings as compared to the star configuration, and thus the star configuration is typically used for direct line connected motor start up so as to avoid power source overloads. Once the motor has started up, the windings are switched to a delta configuration either through an open transition in which power to the motor is removed for a period of time surrounding the transition to avoid contact arcing or through a closed transition where power remains connected to the motor during the transition, typically through the use of added load resistors.
Soft starters were developed to replace the traditional star-delta motor startup, and typically operate by reducing the voltage delivered to the motor during the start up phase. Once the start up phase is complete the soft starter is disconnected from the motor, and the motor is connected directly to the grid. Additionally, soft starters do not support energy savings during operation, nor do they provide an efficient means of braking since they do not participate in slowing down the motor when braking is required. The start up period is extended and peak power consumption during the start up period is excessive.
A frequency converter, which converts the incoming AC line power to a DC voltage, and then reconverts the DC to an AC voltage at a variable output frequency and voltage, provides both rotational speed control and soft starting. Since the actual speed of the motor is dependent on the received power frequency, voltage and load, by adjusting the output frequency fed to the AC motor the speed of the AC motor is controlled. Advantageously a frequency converter can support smooth operation of an AC motor over a continuous broad range of speeds, including speeds down to 1/10 nominal and below. A frequency converter avoids the need for star delta switching or a soft starter by instead starting the motor with a low frequency and low voltage output. Unfortunately, frequency converters are expensive and bulky, and many motors do not require such a broad range of control.
A cycloconverter converts an input AC waveform to an output AC waveform of a different frequency, without requiring an intermediate DC conversion, by synthesizing the output AC waveform from segments of the input AC waveform. Cycloconverters are most often found in very high power output systems such as variable frequency drives exhibiting ratings of several megawatts. The operation of cycloconverters are well known and are available commercially from, among other companies, Siemens AG, Germany.
There is thus a long felt need for a method and apparatus which provide low cost controlled starting, rotational speed control, regenerative braking and power reduction.